Some don't choose to leave, they get demoted. On the other hand, some moderators need to focus more on school or their social lives instead of being glued to a video game
Demotions can happen for a variety of reasons, including breaking a special set of rules and expectations for moderators and inactivity. Resignations usually happen because they want to focus on their real life more, whether it be school or other things.
Remember, in the end, it's a video game, and mods have their own lives. If real life means they cannot perform their duties then they should move on. For demotions or being fired, sometimes it happens and that is unfortunate.
Some of the mods have left caise of the way this place is run, and some cause of all the drama and disagreements and such inside the mod team.